The present invention relates generally to gas turbine engines, and, more specifically, to blade containment therein.
A typical gas turbine engine includes in serial flow communication a fan, multistage axial compressor, combustor, high pressure turbine (HPT), and low pressure turbine (LPT). During operation, air is pressurized in the compressor and mixed with fuel and ignited in the combustor for producing combustion gases which flow downstream through the HPT and LPT which extract energy therefrom for powering the compressor and fan, respectively, through corresponding driveshafts.
The fan, compressor, and turbines each include differently configured rotor blades extending radially outwardly from corresponding rotors or disks which rotate during operation. For various reasons during the useful life of the engine, a rotor blade may fail and separate from its corresponding rotor disk. Centrifugal force will then propel or eject the liberated blade radially outwardly into its surrounding stator case. The different stator cases are configured in various manners for dissipating blade ejection energy for containing the blade and preventing its liberation from the engine.
The various rotor blades are different in size and operate at different rotary speeds and therefore have different amounts of ejection energy when liberated. The different rotor blades also require different surrounding stator cases which experience different operating environments from the relatively cool environments in the fan, compressor, and LPT, to the hottest environment in the HPT.
Since engine efficiency is maximized by minimizing the radial clearance or gap between the radially outer tips of the corresponding blades in their cases, the cases include various forms of blade shrouds surrounding the blade tips for minimizing the clearance therewith while also permitting occasional rubs therebetween without damaging the blades. In a tip rub, the blade shrouds are damaged, and when such damage accumulates, the blade shrouds are replaced in a periodic maintenance outage.
In turbine blade containment, the corresponding turbine cases are correspondingly sized in thickness for dissipating the ejection energy. In the HPT, the blade shrouds provide a significant contribution to blade containment since they are typically relatively thick, cast metal structures having substantial strength.
However, LPT blade shrouds are typically uncooled, light-weight sheet metal constructions having little, if any, significant ability for dissipating ejection energy. A typical LPT blade shroud is an assembly of a sheet metal backsheet having a light weight honeycomb rub strip attached thereto. The backsheet has forward and aft rails which are suitably mounted to corresponding forward and aft mounting hooks extending radially inwardly from the case. The backsheet is thin sheet metal, of about 20 mils thickness for example, for minimizing the weight of the shroud yet providing sufficient rigidity for being mounted to the case and maintaining a preferred clearance with the blade tips. The sheet metal may be locally thickened at one or both of the rails for providing sufficient strength for attachment to the corresponding hooks.
In some configurations, the backsheet may be too thin between its axially separated rails, and is reinforced using a doubler sheet, which is typically another thin piece of sheet metal brazed or otherwise fixedly attached to the outer side of the backsheet.
In either configuration of the LPT shroud, with or without the doubler, the blade containing capability thereof is negligible. Since the doubler, for example, is brazed to the backsheet, the brazing filler is relatively brittle and in a blade ejection event the filler is subject to brittle cracking and decreases the strength of the shroud.
Accordingly, it is desired to provide a LPT blade shroud having blade containment capability.